For modern systems of illumination, whether illumination associated with ambient lighting, image capture, image projection, image viewing, signage illumination and/or projection, etc. it is often desirable to generate light with one or more narrow band spectral components. In particular, it is often desirable to generate narrow band spectra corresponding to one or more of the additive primary-color components red (“R”), green (“G”) and blue (“B”) and/or spectra corresponding to one or more of the subtractive color components magenta (“M”), cyan (“C”) and yellow (“Y”). A full set of narrow band spectra corresponding to the primary colors may be color-balanced to create white light. White light may in turn be filtered to create any other color.
Another example use of a set of primary colors of light is the time-sequencing of each primary color onto a digital micro-mirror device (“DMD”) associated with a Texas Instruments Incorporated Digital Light Processing (“DLP”)™ projection system. Each micro-mirror of a two-dimensional matrix of micro-mirrors on the DMD surface may be separately re-positioned at the start of each primary color time slot to reflect a single pixel of the current primary color into or away from an optical projection system. Doing so produces a projected two-tone pixel image of the current primary color. A series of such two-tone pixel images projected while rapidly sequencing between primary colors are integrated by the human eye to create the illusion of full-color image frames seen as a still or moving picture.
Many systems of illumination require significant light power expressed in lumens. In turn, energy efficiency standards often dictate minimum light generation efficiency expressed in lumens of output light power per watt of electrical input power. Laser light is potentially powerful and efficient to generate. In particular, blue light emitted at approximately 448 nm and ultraviolet (“UV”) light emitted at approximately 405-420 nm are energetic and can be generated by lasers at high efficiencies. To take advantage of this phenomenon and to engineer simpler multi-spectrum illumination systems, light from one or more blue or UV lasers may be used to excite one or more luminescent phosphors coating one or more portions of a surface of a wavelength conversion element such as a phosphor wheel. Various phosphors are available, each capable of luminescing in a particular narrow spectrum when excited by a particular narrow band excitation spectrum of light. This technique may be used to create multiple primary colors from a low etendue, narrow band excitation light source such as a laser.
FIG. 1 is a prior-art schematic diagram of a multi-wavelength light generation apparatus 100 using a low etendue, narrow band excitation light source. The apparatus 100 includes one or more blue light excitation lasers 110 to generate light 115 of the excitation spectrum. The light 115 is collected and collimated in an excitation light lens group 118 to generate an excitation beam 122. The excitation beam 122 is reflected by a dichroic mirror 127 through condensing and collimation optics group 133 and onto a surface of a phosphor wheel 138. Each of various phosphors coated onto various areas of the surface of the phosphor wheel 138 luminesces in a predetermined fluoresced spectrum of light as it is illuminated by the excitation beam 122.
It is noted that the various phosphor-coated areas may be exposed to the excitation beam 122 at different times by locating the areas radially around the wheel surface and rotating the wheel. Doing so may desirably temporally separate the output colors. Each resulting fluoresced spectrum of light emanating from the phosphor wheel 138 (e.g., as represented by light rays 143, 148, and 150) corresponds to a desired output color (e.g., R 153, G 158 and Y 160). The phosphor-emitted light is collected and collimated by the optics group 133 and are passed to the output 165 through the dichroic mirror 127.
One problem with using a dichroic mirror to separate light of the excitation spectrum from light of the fluoresced spectrum is that it may be desirable to include light of a color corresponding to the excitation spectrum (e.g., blue) at the output 165. The latter cannot be accomplished by simply reflecting light of the excitation spectrum from the phosphor wheel surface, because the dichroic mirror is designed to reflect and not pass light of the excitation spectrum. Consequently, such a dichroic mirror-based system may include a separate light source 170 to generate light of a color corresponding to the excitation spectrum. A dichroic mirror-based system may also include an associated optical group 175 on the side of the dichroic mirror 127 opposite the excitation energy source 110. The blue light source 170 emits light of the excitation spectrum at the output 165. Such additional components add cost and complexity.
FIG. 2 is a prior-art schematic diagram of a multi-wavelength light generation apparatus 200 using a low etendue, narrow band excitation light source. The apparatus 200 includes one or more blue light excitation lasers 110, light 115, excitation light lens group 118, excitation beam 122, dichroic mirror 127, condensing and collimation optics group 133, phosphor wheel 138 and output 165, all as described above with reference to FIG. 1. However, the apparatus 200 also includes an opening 210 in the phosphor wheel (e.g., a slot along a radius) to pass light of the excitation spectrum (e.g., blue) at a time when the color corresponding to the excitation spectrum is desired. A series of mirrors (e.g., the mirrors 215, 220 and 225) create a “wrap-around” path 230 to direct light of the excitation spectrum to an optical group 235 associated with the wrap-around path 230. Collimated light of the excitation spectrum is subsequently reflected by the dichroic mirror 127 to the output 165. The wrap-around path 230 may increase the overall size of the apparatus 200.